Method of making resistance devices



May 11, 1937. R. o. GRlsDALE METHOD OF'MAKING RESISTANCE DEVICES Fild Jan. 2., 1936 Y m l M/// Patented`May 1l, 1937 4UNITED7`s'rAfras PATENT OFFICE METHOD OF MAKING RESISTANCE DEVICES Application January 2, 1936, Serial No. 57,180

6 Claims.

This invention' relates to the preparation of electrical resistances from silver sulphide and more particularly to electrical resistances formed 15 the direction from which this temperature is.

approached.

Another object of the invention is to produce an electrical resistance unit having a large negative temperature coeflicient of resistance Whose 20 resistance is independent of frequency or time and of voltage except insofar as an increase of voltage resulting in an increased power dissipation may cause a consequent decrease in resistance.

A still further object of the invention is to provide methods of making electrical resistances possessing the characteristics set forth above.

Many materials are known to have negative temperature coeicients of electrical resistance; and, in fact, this property is possessed to some degree by most non-metallic conductors and in particular by the so-called semi-conductors.

Most of these `materials, however, are not satisfactory for use in electrical apparatus either because their temperature coefficients of resist ance are not great enough, or because their specic electrical resistances are not of thecorrect magnitude, or because they are not stable or reversible, or because of their chemical natures, 40 they are not readily fabricated into suitable forms.

Silver sulphide possesses a combination of properties which make it uniquely suitable for use in electrical devices as a means of compensating for resistancel changes in metallic conductors which possess positive temperature coecients of resistance or as'a control or compensatory device to operate when the change in temperature attains some predetermined value.

I have determined that by the following procedures resistance units which are suitable for use in electrical devices of the above-mentioned varieties can be prepared.

fai

scribed in detail, silver sulphide resistance units are formed by depositing silver sulphide on base members. These base members may be rods or plates of glass, ceramic, or other insulating materials or they may be of metals or alloys. The base members coated with silver sulphide may then be heat treated in an atmosphere of dry nitrogen to remove volatile impurities from the silver sulphide.

According to my method, silver sulphide film can be deposited directly on glass, ceramic, or metal supports by the chemical decomposition of solutions of complex sulphur bearing silver salts. For example, when a small quantity of a dilute solution of sodium hydroxide is added to solution of silver nitrate or acetate in combination with thiourea, at 30 C., silver sulphide is deposited on clean surfaces which have been immersed in the solution just after the addition of the sodium hydroxide. sulphide may then be heat treated in an atmosphere of dry nitrogen to drive off impurities such as excess sulphur and to set the deposited material so that the resistance characteristic will not change with age or normal use.

In Fig. 1 a support or base member I0 is shown. This support may be of glass, ceramic, or other insulating material or of a metal or an alloy. The surfaces of the support I0 should preferably be free from dirt or grease, such materials being removed by mechanical action or by suitable chemical agents such as the mineral acids, ether, benzine, alcohol, or other organic solvents.

In Fig. 2 the support I0 is suspended by a string II, or other supporting device in a vessel I2, containing a solution of a, complex sulphur bearing silversalt I3 to which has beenadded a small quantity of sodium hydroxide solution, the support I0 being completely immersed in the solution I3. 'I'he vessel I2 and the solution I3 are preferably maintained at a temperature of 30, C. by a source of heat I 4. The object or support I0 which is to be coated is preferably immersed in the solution I3 immediately after the small quantity of alkali hydroxide has been' added to this soluton. The alkali hydroxide is added in order to catalyze the chemical reaction which results in the deposition of the silver sulphide. This reaction is completed in a relatively short time, an immersion of eight minutes being sullcient to produce a satisfactory coating. If desired, successive deposits of silver sulphide may be produced on the same base member by repeating the above procedure.

After the support. I0 yhas been subjected to The deposit of silver sulphide coated unit I5 shown in Fig. 3.

An alternative method of producing silver sulphide film on supporting base members is the following. I have found that such nlm can be produced by immersing the object to be coated in a solution of silver nitrate or acetate in combination with sodium thiosulphate or sodium thiocyanate. When such a solution is employed it is preferable that it be heated to a temperature between 80 C. and 120 C. In order that the silver sulphide obtained by this method will adhere well to the base member, it is preferable that it be thoroughly cleaned 4and then washed in a fresh dilute solution of stannous chloride. The time required to produce a satisfactory film by this method will depend upon the temperature f the solution and may va, y from a few minutes to an hour or longer.

To drive off excess sulphur and impurities the unit I5 may be heat treated in an atmosphere of pure dry nitrogen. This may be done by suspending the unit I5 as shown in Fig. 3 in the vessel I6 supported in the heating chamber I1 heated by the heating coil I8. Pure dry nitrogen is supplied to the vessel I6 through an inlet pipe I9 and is fed into the lower portion of the vessel I6. A plug is provided in the top of the vessel I 6. The plug 20 is apertured to accommodate the inlet pipe I9 and to support an outlet pipe 2| the outlet 2I serving as an outlet for the' nitrogen and gases developed in the vessel I 6. 'I'he temperature ofthe interior of the vessel I6 is indicated by the thermometer 22.

I have found that if a temperature of about 200 C. is maintained in the vessel I6 and the unit I5 is allowed to remain in the vessel I6 for about one hour in the atmosphere of nitrogen, that the excess sulphur -is removed from the silver sulphide coated unit I5 and the unit I5 will possess the required resistance characteristic and be stable in service.

After the unit I5 is removed from the vessel I6 it is ready to have the terminals or lead-in wires secured thereto.

Due to the character of the resistance unit I5 the well-known method of mounting or securing connections to resistances or the like cannot be used. It is therefore necessary to provide a different method of making connections to this resistance unit.

As shown in Fig. 4 the central portion of the resistance is masked by means of tape or some other similar material 23. The exposed portions 24 and 25 on the ends of the resistance are then sprayed with a conducting metal 26 by means of the gun 21 preferably in the manner of the wellknown Schoop process. A coating of conducting metal is therefore formed at 28 and 29 on the ends of the resistance I5 as shown in Fig. 5. Lead-in wires 30 and 3| are then secured to the respective ends 28 and 29 of the resistance I5. It will be understood that I do not limit myimmersing in the solution I3 it becomes the silver self to the precise apparatus herein disclosed as the same is only used in an illustrative sense and that the invention is only limited to the scope of the appended claims.

What is claimed is: 5

l. A method of producing resistor units having a large negative temperature coeicient of resistance for use in electrical circuits which comprises the deposition on a supporting member by chemical reaction of solutions of silver nitrate 10 in combination with thiourea and sodium hydroxide, and heat treating the coated supportingl member in an atmosphere of pure dry nitrogen. v

2. A method of producing resistor units having a large negative temperature coeflicient of ll resistance for use in electrical circuits which comprises the deposition on a supporting member by chemical reaction of solutions of silver nitrate in combination with thiourea and sodium hydroxide, and subjecting the coated supporting gg member to a temperature of about 200 C. in an atmosphere of pure dry nitrogen.

3. A method of producing resistor units having a large negative temperature coeiiicient of resistance for use in electrical circuits which comg5 prises subjecting a support to a bath of a solution of complex sulphur bearing silver salts in combination with alkali hydroxide, and heat treating the coated support in an atmosphere of pure dry nitrogen. :l0

4. A method of producing resistor units having a large negative temperature coeicient of resistance for use in electrical circuits which comprises subjecting a support to a bath of a solution of complex sulphur bearing silver salts to obtain a coating of silver sulphide on the support, and then subjecting the coated support to a temperature of about 200 C. for about one hour in an atmosphere of pure dry nitrogen.

5. A method of producing resistor units for 4o use in electrical circuits which comprises adding a small quantity of a dilute solution of sodium hydroxide to a solution of silver nitrate in combination with thiourea maintained at a temperature of about 30 C., immersing abase mem- 45 ber in the solution of silver nitrate immediately after adding the dilute solution of sodium hydroxide, removing the base member from the l solution after a sufcient coating of the silver solution has been obtained on the base member and subjecting the coated base member to a temperature of about 200 C. in an atmosphere of dry nitrogen.

6. A method of producing resistor units for use in electrical circuits which comprises immersing a clean support in a solution of silver acetate in combination with sodium thiocyanate maintained at a temperature between C. and 120 C., removing the support from the solution when a required coating is obtained on the support and subjecting the coated support to a temperature of about 200 C. in an atmosphere of dry nitrogen.

RICHARD O. GRISDALE. 

